The Assignee of the present invention manufactures constant speed drives (CSDs) and integrated drive generators (IDGs) for use in generating 400 Hz. three-phase alternating current in airframes. FIGS. 1 and 2 illustrate conceptually a block diagram of a CSD or IDG of the type manufactured by the Assignee of the present invention. With reference to FIGS. 1 and 2 in which like parts are identified by like reference numerals, a variable speed power take-off 10 is driven by an airframe propulsion engine which varies in speed in direct proportion to the speed of the propulsion engine. When the unit of FIGS. 1 and 2 is an IDG, the power takeoff 10 drives a hydromechanical transmission 18 comprised of a hydraulic pump and motor (not illustrated) contained within the case 12. The transmission adds or subtracts shaft RPM from the power take-off 10 to produce a constant velocity output on shaft 20 which drives a three-phase generator 22 at a constant velocity to produce 400 Hz. three-phase alternating current. When the unit of FIGS. 1 and 2 is a CSD, the input drives a hydromechanical transmission within the case 12 (not illustrated) comprised of a hydraulic pump and motor (not illustrated) to produce a constant velocity output on shaft 14 which drives a three phase generator 16 at a constant velocity to produce 400 Hz. three phase alternating current. An IDG differs from the CSD in that the constant speed drive transmission 18, constant speed shaft output 20 and three-phase generator 22 are each contained within the case 12 instead of the generator 16 of the CSD being external of the case 12 of the transmission. The overall function of a CSD and an IDG is to perform the identical function of generating three-phase 400 Hz. electrical power in an airframe.
New IDGs operate at elevated temperatures. Ester based synthetic oils are used for hydraulic power, cooling, and lubrication in a CSD or an IDG. New airframes have IDGs which produce a high electrical power output requiring the IDG to dissipate heat as a consequence of losses in the constant speed drive transmission and the generator. Additionally, many industrial applications, such as mobile hydraulic systems and industrial gears, for example, utilize mineral oils for lubrication and cooling as well as for working fluid. In all these types of systems, tiny metallic wear particles are formed through the normal operation of the transmissions, gears, hydraulic cylinders, etc. These wear particles are then carried by the oil within the oil circuit. Although some of the wear particles are filtered out of the circuit by the oil filter, many of these wear particles are so small that they are not filtered out by the oil filter. These tiny wear particles are simply carried in suspension by the oil without effect to the operating system.
The operation with either synthetic ester based oils or mineral oils, however, causes the formation of organic acids through the process of hydrolysis. This process occurs at all operating temperatures, but is accelerated at elevated temperatures. A chemical reaction then occurs with these organic acids and the tiny metallic wear particles within the oil circuit to form a metallo-organic soap and/or insoluble products which circulate within the oil contained within the circuit. Unlike the very small wear particles which may be carded in suspension without effect, the metallo-organic soap particles have side chains which bond to other metallo-organic soap particles forming large aggregations which then become trapped in and occlude the interstices of the filter, thereby shortening the service life of both the oil and filter. This process occurs in systems which utilize synthetic ester based oils as well as in systems utilizing mineral oils.
To help illustrate this problem and the application of the following invention, the following discussion will center on an aerospace system which realizes the formation of metallo-organic soaps through the reaction of organic acids with the metallic wear particles. However, the following is to be taken as an exemplary system only, and in no way should this exemplary system be read as the only system to which the invention may be applied. Indeed, the instant invention is applicable to any system which contains a source of metallic wear particles in the oil circuit including, but not limited to, aerospace and industrial applications which utilize either synthetic or mineral oils for lubrication, cooling, or as a working fluid.
The CSD or IDG as illustrated in FIG. 1 contains an oil circuit 24 which contains a filter 26 external to the case or internal to the case as illustrated in FIG. 2. The oil circuit 24 containing the filter 26 cools and cleans the oil within the CSD or IDG. The oil circuit 24 additionally includes an oil pump 28 which scavenges oil within the case 12 to pump the oil 30 to the filter 26 where the oil is filtered to remove entailed solids. The output of the filter 26 is applied to an oil cooler 28 which dissipates the heat picked up by the oil from operation of the hydraulic pump and motor and gearing contained within the transmission of the CSD and the hydraulic pump and motor plus electric generator of the IDG.
The reaction of the lubricants to form organic acids in the oil 30 contained within the case 12 causes the metallo-organic soaps and/or the insoluble products to plug the filter 26 in a relatively short time of operation of the transmission such as 300 to 750 hours as described above. Some aircraft have a specification of a minimum time of 1,200 hours between change of the filter 26 and the oil 30 which is not met by use of oils which are approved generally for airframe applications such as, for example, MIL-L-23699 and MIL-L-7808.
The aforementioned oils, as well as many mineral oils for use in industrial or ground and sea based applications, do not contain adequate additives to prevent the formation of metallo-organic soaps and/or solids which cause the filter to become plugged or otherwise not fully operational requiring replacement sooner than its specified service life. The specifications of many of these oils do not require sale with additives especially suited for preventing the formation of metallo-organic soaps and/or solids. In the aerospace industry, for example, a consequence of the overall relatively small quantity of oil which is sold for use in the CSD or IDG transmissions, is that no manufacturer of oil has been willing to conduct the necessary tests to obtain approval of an oil containing adequate additives for preventing the formation of the aforementioned metallo-organic soaps and solids to permit the filter and oil to be used for their specified service life. The currently available oils break down into organic acids through hydrolysis that chemically attack the wear particles and the metal casing 12 of the CSD or IDG. Formulating new or revised oils that will not form organic acids and/or attack the wear particles or CSD or IDG metal casing would require extensive field/flight evaluations with approval taking from two to eight years with the norm being around six years. The Assignee has requested oil companies to develop a specific IDG/CSD oil for eleven years without obtaining any interest on the part of oil companies to do so.
Liquid oil additives are known which may be mixed with oils to neutralize acids. For example, see U.S. Pat. Nos. 2,889,338, 3,941,709, 3,969,254, 3,976,585, 4,189,388, 4,226,732, 4,461,713, 4,568,474 and 4,943,383. Additionally, metal deactivating compounds are known which may be mixed with oils to provide thin layer coating of the metallic wear particles to insulate them from attack from the organic acids.
Servicing of an oil circuit includes regular oil and filter changes. Those regular oil and filter changes include changing the filter 26 and draining the oil 30 from the case 12. For the exemplary system described above, an airplane mechanic places a new filter in the CSD or IDG and fills the case with new oil 30. As a consequence of the problem involving formation of metallo-organic soaps and/or insoluble solids, the recommended service intervals are so short as to cause airlines to want to have longer service intervals between routine oil and filter changes to lessen the overall operational costs of CSDs or IDGs. The servicing of an oil circuit for other applications is similar to this exemplary system, as is the desire to reduce overall operational costs of operation by having longer service intervals.
In the aerospace industry for example, the airlines desire an oil filter which provides longer service intervals such as up to 3,000 hours in order to lessen the operational cost of the CSD or IDG. A filter having a service life of 3,000 hours is currently not available as a result of the problems of the prior art discussed above. Additionally, such an oil filter which provides longer service intervals is desirable in industrial and other ground based applications as well.
FIGS. 3 and 4 illustrate a prior art oil filter of the type illustrated in FIGS. 1 and 2. The oil filter 26 has an end cap 40 which has an outlet for discharging oil pumped under pressure by the oil pump 28. An O-ring seal 42 provides suitable sealing between a fitting mating with the end cap 40. Oil flows into the cylindrical chamber 44 within the filter 26 by flow radially inward through a pleated filtering media 46 as described below, an inner perforated cylindrical support tube 50 having apertures 52. The pleated filtering media 46 surrounds the inner perforated support tube 50 and prevents radially inward deflection of the filtering media 46 caused by radial inward flow of pressurized oil. The pleated filtering media 46 contains interstices for trapping solid particles flowing within the oil. Oil flows inward through the filtering media 46, inner perforated support tube 50 and from the outlet at the end cap 40 to the oil circuit 24. The filtering media 46 and inner support tube 50 are glued to end cap 54 by a suitable adhesive such as epoxy glue. Similarly, end cap 40 is attached to the filtering media 46 and inner support tube 50 by epoxy glue.
The filtering media 46 is comprised of a sandwich of four (more or less depending on specific construction) pleated layers illustrated in FIG. 3 and in detail in FIG. 4. The media 46 is formed into a cylinder 56 with pleats 58 extending longitudinally along the length of the filter. The cylinder 56 completely encircles the cylindrical chamber 44. The inner layer 60 (with reference to the cylindrical chamber 44) is an aluminum, stainless, or other metal type screen. A first intermediate layer 62, which is a nylon or polyester scrim, contacts the inner layer 60. A second intermediate layer 64, which is a fiberglass filtering media, contacts the nylon or polyester scrim 62. The outer layer 66, which is an aluminum, stainless steel or other metal type screen, contacts the fiberglass filtering media 64. The thickness and surface area of the filtering media determine the filtration produced by the filter 26. The aforementioned metallo-organic soaps and/or solids occlude the interstices of the filter.
In the prior art, the filter 26 is packaged dry within a suitable package for preventing its exposure to dirt. The mechanic removes the filter 26 from the package and places it within the oil filtration circuit 24 while the filter 26 is dry. No additives are contained within the filter in the prior art. Oil is added to the case 12 during changing of the oil or to top off the case between oil changes.
U.S. Pat. Nos. 2,392,901, 2,785,805, 3,224,592, 4,751,901, and 4,886,599 each disclose filters which have been coated with solids or additive packages to neutralize the formation of acids within the oil being filtered or to replace the additives which are already included within the oil. None of the aforementioned patents, however, suggest the usage of a liquid additive which wets the interstices of the filter prior to use with a quantity of liquid additive that is washed from the interstices into solution with the oil during filtration by the filter to form a mixture of the oil and additive which reduces the formation of metallo-organic soaps and solids that are retained within the interstices of the filter.